1. Field of the Invention
This invention relates to a measuring method and a measuring instrument for measuring, for example, the size or the surface shape of an object (to be measured) with a probe.
2. Description of the Related Art
In a coordinate measuring machine, for example, a three-dimensional measuring instrument, a touch trigger probe is allowed to contact an object (to be measured) during the relative movement of the touch trigger probe and the pre-measured (throughout this application the word "pre-measured" means "to be measured") object in three-dimensional directions. Each coordinates of axes (of three-dimensional directions) is read in response to a contact-detection signal output from the touch trigger probe at the contact between the touch trigger probe and the pre-measured object. And then the size or configuration of the pre-measured object is obtained from these coordinates.
Conventionally, two types of touch trigger probe are known for using them in the coordinate measuring machine or the like: a touch trigger probe having a structure such that three pins, formed with a measuring point integrally, are supported with six balls or pins, that is to say a structure such that the measuring point is supported by a six-point contact type seating system; and a touch trigger probe having a structure such that a vibrated measuring-point is contacted on a measured object and the contact with the pre-measured object is detected by sensing the decrease of the vibration.
As shown in FIG. 13, the former touch trigger probe includes, for example, a probe body 4, a measuring point 1 having a contacting ball at the end which is supported in the probe body 4 by the medium of the six-point contact type seating system 8, and a pressing means 7 for forcibly pressing the measuring point 1 to the seating system 8, such as a spring or the like.
The seating system 8 consists of: a movable portion 2 secured on the base end of the measuring point 1; three pins 3 projected from the circumferential face of the movable portion 2, perpendicular to the axis of the measuring point 1, in radial at 120 degrees about the axis of the measuring point 1; and three V-shaped engaging portion 6 each of which is a combination of two pins (or balls) arranged in a V shape, and is secured on a bottom wall 5 of the probe body 4 to correspond to the pin 3.
In the above structure, when the movable portion 2 is pressed toward the bottom wall 5 of the probe body 4 by the pressing means 7, the movable portion 2 stands at a predetermined position. Here, the pin 3 provided on the movable portion 2 is in contact with the two points of the V-shaped engaging portion 6, in total, the pins 3 are in contact with the six points, so that this system is called the six-point contact type seating system.
As shown in FIG. 14, the later touch trigger probe includes a stylus holder 12, a vibrator 14 as a measuring point which is supported in the stylus holder 12 with a pin 18 at the central area thereof, and two piezoelectric elements 16 placed in the periphery of a node of vibration (i.e. a portion of vibrating object which does not vibrate in view of certain frequency) of the vibrator 14 and secured to a recess 20 with an adhesive or the like.
The vibrator 14 has a contacting portion 14A connecting the pre-measured object at the end and a balancer 14B, having the same weight as the contacting portion 14A, at the other end, and is supported in the stylus holder 12 at the node of vibration. The balancer 14B is provided in order to keep the vibrator 14 in weight-balance not to deviate the node of vibration from the center of the vibrator 14 in the resonance by reason of the contacting portion 14A placed at the end of the vibrator 14, and in order not to receive the moment of rotation about the fulcrum when the touch trigger probe moves in a perpendicular direction to the vibrating direction of the vibrator 4 and receives the acceleration.
As shown in FIG. 15, the piezoelectric element 16 is split into a vibration applying electrode 16A for applying vibration to the vibrator 14 in a reciprocating vibration and a detecting electrode 16B for detecting the reciprocating vibration of the vibrator 14 as an electric AC signal. The vibration applying electrode 16A is connected through a signal line 22A to a drive circuit 24 for vibrating the vibrator 14. The detecting electrode 16B is connected through a signal line 22B to a detection circuit 26 for detecting the vibration of the vibrator 14. A signal from the detection circuit 26 is send through a signal line 22D to the drive circuit 24. Thereby, a feedback loop is designed to amplify an electric signal, fetched from the detecting electrode 16B, in the detection circuit 26 and the drive circuit 24, and sent it to the vibration applying electrode 16A.
The detection circuit 26 is connected to a touch trigger signal producing circuit 28. In the touch trigger signal producing circuit 28, an AC signal from the detecting electrode 16B is full-wave rectification, and the rectified signal is compared with a reference value after being converted to a DC signal through a low-pass filter, and then a touch trigger signal is sent out at the time when the detecting signal reaches the reference value.
In the conventional touch trigger probe, however, any structure is at a disadvantage in that a measurement error is contributed by a relative moving velocity at the moment of contact with the pre-measured object, namely, the contact velocity.
In the former touch trigger probe (shown in FIG. 13), the measurement error is large when the relative moving velocity to the pre-measured object is especially small. As one of the reasons, the contact velocity at the instant of contact between the touch trigger probe and the pre-measured object is generally approximately from 3 mm/s to 20 mm/s. It has been shown that when the touch trigger probe contacts the pre-measured object at a speed exceeding the range, the measured value shows a comparatively stable value, but when the touch trigger probe contacts the pre-measured object at a speed below the range, a gap between the coordinates, fetched when a contact-detection signal is generated, and the actual coordinates is produced.
In this point, there is no obvious ground. In the structure of the touch trigger probe shown in FIG. 13, when the measuring point 1 is displaced by contacting the pre-measured object, at least one of six pins (or balls) is in the non-contact state, and this state is detected electrically. When the measuring point 1 is relatively displaced in a low velocity, the amount of flexure prior to separation of the contact is concluded to be larger.
On the other hand, in the later touch trigger probe (shown in FIGS. 14 and 15), the contact-detection sensitivity is higher compared with that in the former touch trigger probe, so that the measurement error in the low-speed area seldom tends toward increase. However, the detection and drive circuits subsequent to the piezoelectric elements are complicated, so that an elapsed time from the contact between contacting portion 14A of the vibrator 14 and the pre-measured object to the output of the touch trigger signal (the contact-detection signal) is longer, therefore the elapsed time causes the measurement error.
For example, as show in FIG. 16, from the contact between the contacting portion 14A of the vibrator 14 and the pre-measured object, when the contact-detection signal is generated after an elapsed time T1, a coordinate value P1, which is .DELTA.P distance from an coordinate value P0 at the moment of actual contact between the connecting point 14A and the pre-measured object, is determined as the coordinate value at the moment of actual contact.
The amount of correction can be defined for correcting the amount of deviation .DELTA.P in advance if the measuring speed is uniform, but the amount of deviation changes in response to the changing of the measuring speed. Especially, in the measuring instrument in which a high speed and high precision are essential, the measurement should be carried out by changing the measuring speed, so that the correction is not carried out rightly.
It should be mentioned that the above disadvantage occurs not only in the touch trigger probe but also in a non-contacting probe.
For example, where the size of the pre-measured object is measured with a non-contacting probe, having an optical displacement sensor for moving an objective lens close to and apart from a measured face of the pre-measured object to focus a focal point of the objective lens on the measured face, or a non-contacting probe of a camera or the like, relative coordinates of the prove and the pre-measured object are fetched when a distance between the non-contacting probe and the pre-measured object is fixed. In this time, a disadvantage that the measurement error is contributed by the relative moving velocity of the prove and the pre-measured object occurs.
It is an object of the present invention to resolve the conventional disadvantages and to provide a measuring method and a measuring instrument, capable of achieving a high-accurate and efficient measurement and of correcting a biased error caused by a measuring speed even in a complicated measurement which requires changing the measuring speed in response to the substance of measuring.